1. Field of the Invention
The present invention relates to an optical modulator and an optical transmitter.
2. Description of the Related Art
In recent years, an optical waveguide device in which an optical waveguide is formed in an electro optic crystal, such as a lithium niobate (LiNbO3) substrate and a lithium tantalate (LiTaO2) substrate is used as an optical modulator in high-speed optical transmission systems of, for example, 40 Gbps. Japanese Patent Application Laid-Open Publication No. 2003-233044 teaches such a device. The optical modulator using this optical waveguide device is formed by thermally diffusing a metal film on a portion of an electro optic crystal substrate or effecting proton exchange in a benzoic acid after patterning to form an optical waveguide followed by provision of a signal electrode near the formed optical waveguide.
FIG. 34 is a plan view depicting a conventional optical modulator configuration. A conventional optical modulator 3400 shown in FIG. 34 is a Mach-Zehnder type optical modulator that modulates the intensity of light. A Mach-Zehnder type optical waveguide 3420 including a branching unit 3421, parallel waveguides 3422 and 3423, and a coupling unit 3424 is formed on an electro optic crystal plate 3410. In addition, a signal electrode 3430 is formed along the parallel waveguide 3422. Moreover, ground electrodes 3440 are formed along the parallel waveguide 3423.
In a section where the parallel waveguide 3422 and the signal electrode 3430 interact, a refractive index of the parallel waveguide 3422 changes due to the effect of an electric field of microwaves passing through the signal electrode 3430. As a result, light passing through the parallel waveguide 3422 is subjected to phase modulation according to the microwaves passing through the signal electrode 3430. Light output from the coupling unit 3424 is subjected to intensity modulation according to a phase difference between the light respectively passing through the parallel waveguides 3422 and 3423.
In the conventional art, however, lengths of the parallel waveguides 3422 and 3423 change with temporal changes, such as a variation in temperature, thereby causing operating point drift and resulting in a problem in that modulation characteristics deteriorate. Operating point drift is a phenomenon in which an optical length until each branched light is coupled is shifted and the phase difference between the branched light in the coupling unit 3424 fluctuates, thereby degrading the optical signal output from the coupling unit 3424. As a countermeasure, applying bias to either one of the branched lights to correct the operating point drift may be considered.
FIG. 35 is a plan view depicting a configuration of an optical modulator that corrects operating point drift. As shown in FIG. 35, the optical modulator 3400 includes a correction circuit 3500 in addition to the configuration of the optical modulator 3400 shown in FIG. 34. The correction circuit 3500 includes a photo diode (PD) 3510, an auto bias control (ABC) circuit 3520, and a biasT 3530.
When the optical modulator 3400 is an OFF state, the PD 3510 receives radiated light output from the coupling unit 3424, converts the radiated light into an electrical signal, and outputs the electrical signal to the ABC circuit 3520. The ABC circuit 3520 determines a bias voltage to be fed back to the signal electrode 3430 according to the electrical signals output from the PD 3510, and supplies the determined bias voltage to the signal electrode 3430 via the biasT 3530.
However, there are problems associated with the optical modulator 3400 shown in FIG. 35 in that as a result of providing the correction circuit 3500 therein, control is complicated, power consumption increases, and the size and cost of optical modulator increases. Particularly, in recent years, conventional communication of 10 Gb/s is moving toward high speed communication of 40 Gb/s. Components used in the correction circuit 3500 for 40 Gb/s are expensive, or have problems in that their characteristics are insufficient or they are large in size.
Meanwhile, in terms of a optical modulator configuration that does not generate the operating point drift, a configuration has been proposed in which each branching unit of the optical coupler is connected by a turnback optical fiber to thereby shift the phase while allowing each of the branched lights to pass in the turnback optical fiber in opposite directions (refer to IEEE Photonics Technology Letters, Vol. 18, No. 11, June 2006, p. 1252; IEEE Photonics Technology Letters, Vol. 4, No. 8, August 1992, p. 855; Electronics Letters, Vol. 32, No. 6, March 1996, p. 547).
In this conventional art, however, since the optical coupler, the turnback optical fiber, and the phase modulator are discrete parts, there is a problem that practical application is difficult in terms of mounting, size, and cost. Specifically, since each branching unit of the optical coupler and the phase modulator are connected by the turnback optical fibers, respectively, it is difficult to mount them, leading to an increase in manufacturing cost.
Additionally, since it is necessary to respectively provide connection portions that mutually connect the optical coupler, the turnback optical fiber and the phase modulator, the optical modulator increases in size, leading to increased manufacturing cost. Further, since the optical length from the optical coupler to the phase modulator is adjusted, it is necessary to accurately adjust the length of the turnback optical fiber of each portion. Still further, it is necessary to match polarization angles of the branched light passing through the turnback optical fiber. For this reason, mounting is difficult, leading to an increase in manufacturing cost.
The present invention solves the problems, and aims at providing an optical modulator and an optical transmitter that can achieve a reduction in size and cost of the optical modulator with a simple configuration, while improving modulation characteristics.